1. Field of the Invention
The present invention relates to an electric double-layer capacitor using a certain carbon material and, more particularly, to a carbon acting as an active material used in polarized electrodes that are employed in an electric double-layer capacitor. The invention relates also to a method of fabricating the aforementioned carbon, a method of selecting the constituent materials of an electrolyte solution for use in an electric double-layer capacitor, a method of fabricating such an electric double-layer capacitor, and its usage.
2. Description of the Related Art
Electric double-layer capacitors (also known as supercapacitors, ultracapacitors, pseudocapacitors, hybrid capacitors, or electrochemical capacitors) capable of being charged and discharged with large currents are promising as electric power storage devices that are frequently charged and discharged such as batteries for electric vehicles, auxiliary power supplies for solar batteries, and auxiliary power supplies for wind generators. Therefore, there is a demand for an electric double-layer capacitor having a high energy density, being capable of being quickly charged and discharged, and having excellent durability (see, for example, “Fourth Symposium On State-Of-The-Art Techniques Regarding EV/HEV: Present Situation Of Capacitor Technologies And Problems To Be Solved”, Executive Committee On “International Symposium On State-Of-The-Art Techniques Regarding Batteries For Electric Vehicles”, Nov. 8, 1999).
In an electric double-layer capacitor, a pair of polarized electrodes are located opposite to each other via a separator within an electrolyte solution to form positive and negative electrodes. In principle, electric charge is stored in an electric double-layer formed at the interface between each polarized electrode and the electrolyte solution. Accordingly, only activated carbon having a large specific surface area has been used in the past because it has been considered that the capacitance of the electric double-layer capacitor is roughly proportional to the surface area of the polarized electrodes.
In other words, a substance having a maximum surface area per unit weight has been selected as the material of the polarized electrodes because the electric double-layer capacitor is formed at the interface between a conductive material in solid phase and an electrolyte solution. In practice, the aforementioned “per unit weight” should be read as “per unit volume”, since the space consisting of thin holes between the carbon particles forming the electrode and inside the carbon particles is filled with the electrolyte solution and the weight of the electrolyte solution is added.
Such activated carbon is fabricated by carbonizing a carbonaceous material at a temperature lower than 500° C. and then activating the material. The activating operation is carried out, for example, by heating the material to 600° C. to 1000° C. within an atmosphere of water vapor, carbon dioxide, or the like or mixing zinc chloride, potassium hydroxide, or the like into the material and heating the mixture within an inert atmosphere. Micropores are created by the activating process, thus increasing the specific surface area of the activated carbon. Specific surface areas measured by a BET gas absorption measurement method are generally about 1,000 to 2,000 m2/g.
On the other hand, it has already been proposed to provide a novel polarized electrode made of a carbon having a specific surface area of about 300 to 400 m2/g and an interplanar spacing (inter-layer distance) (d002) of 0.365 to 0.385 nm in order to obtain an electric double-layer capacitor having a large capacitance (Japanese patent laid-open No. 11-317333). In particular, an easily graphitizable carbon containing a large amount of crystallites of a multilayer graphite structure having a well developed carbon mesh surface structure is prepared. This carbon is dry distilled at 700° C. to 850° C. to remove the volatile components. The remaining material is thermally treated together with KOH at 800° C. Using this carbon, an improvement of about 40% in capacitance over the electric double-layer capacitor using the prior art activated carbon has been accomplished.
However, the electric double-layer capacitor proposed by the present applicant as mentioned above suffers from some difficulties. That is, the capacitance decreases during repeated use. This involves generation of gas. Also, the internal resistance increases. In addition, the mechanism of capacitance production is not understood.